Two predominant television standards for video signals are PAL (Phase Alternation by Line) and NTSC (National Television Systems Committee). Encoded according to one of these standards, color television broadcasts are typically transmitted as composite video signals that include a brightness signal (luminance, luma or Y) and a color signal (chrominance, chroma or C).
To produce the composite video signal, a modulated color signal (color sub-carrier) is added to the luminance signal prior to transmission. The chrominance signal occupies the same frequency spectrum as the high frequency luma signals, so color television receivers include an Y/C separator circuit for separating the composite video signal into its luminance and chrominance components. However, Y/C separators often permit crosstalk from the luminance into the chrominance (cross color) and of chrominance into luminance (cross luminance). Y/C crosstalk generally degrades the quality of displayed video pictures.
Many techniques for separating composite video signals into luminance and chrominance components exist. For instance, a band-pass filter used in conjunction with a notch or comb filter can separate the chrominance component from a composite signal. However, in addition to passing the chrominance signal, a notch/band-pass filter will pass high frequency luminance energy in the chrominance pass band. Therefore, some luminance appears as cross-color in a displayed picture. For instance, a vertical striped pattern on a displayed article of clothing can have a spatial frequency creating a high frequency luminance component, which appears as cross-color in a separated chrominance signal. This cross-color usually appears in a video picture as a colored rainbow superimposed on the reduced-amplitude striped pattern.
Although the notch/band-pass filter combination has drawbacks, it is useful under certain conditions. For example, the filter combination is effective in picture regions lacking high luminance frequencies in the horizontal direction while containing high frequency chroma information in the vertical direction. In these regions, the comb filter described below degrades pictures with cross luma.
Comb filtering is another technique for Y/C separation. Comb filtering usually provides a considerably better component separation compared to the above-described notch/band-pass filter combination. Although conventional comb filters provide improved separation, crosstalk between the chrominance and luminance still occurs. Comb filters are therefore most effective in pictures having a flat field of color or high luminance frequencies in the horizontal direction. However, when spatial discontinuities in the vertical direction occur in a video picture, a conventional comb filter may inadequately separate chrominance and luminance components, causing undesirable artifacts in the displayed picture.
Accordingly, there exists a need for an improved filter for separating the chrominance and luminance components of a composite video signal.